Prior art lead-acid storage batteries for electric vehicles have so far been used in golf carts, lawn mowers and the like and batteries with discharge characteristics of about 1 CA have served the intended purpose sufficiently.
However, electric cars among electric vehicles are required to show a driving performance (acceleration and gradability) comparable to that of gasoline-driven cars. Therefore, lead-acid storage batteries for electric cars are required to maintain extremely severe high rate discharge characteristics (output characteristics) over a wide range of depth of discharge when compared with the conventional lead-acid storage batteries for electric vehicles.
In addition, durability as represented by high rate discharge cycle life is also considered important. All in all, an improvement in the foregoing has been the problem involved with lead-acid storage batteries for electric cars.
A variety of methods for improving the output characteristics and high rate discharge cycle life have been studied.
For example, a development work has been carried out on a grid with an excellent current collecting efficiency. Further, such attempts as increasing reaction areas of electrodes and the like have so far been pursued by increasing the number of positive and negative electrodes through the use of thinner electrodes.
In general, there are a casting method and also an expanding method that uses a belt shaped sheet of lead or lead alloys prepared in advance by rolling and the like in the methods for producing a grid used in a lead-acid storage battery.
In the case of a grid produced by the casting method, it is possible to design a variety of grids having excellent current collecting efficiencies according to the design of casting dies. However, this casting method shows poor productivity and further makes it very difficult to produce thinner grids that are needed to improve the output characteristics of a lead-acid storage battery. For the production of thinner grids, various methods such as a vacuum casting method and the like have been developed. Since batteries for electric cars are frequently in use, the batteries for electric cars are required to have a maintenance-free performance. Use of an alloy of lead and calcium in a grid is desirable to realize this maintenance-free performance but such a grid as above shows poor castability, thereby imposing limitations on the thickness of electrodes.
On the other hand, the expanding method is to apply slitting on a belt shaped material and then apply an expanding process to the slit belt shaped material, thereby producing a sheet with mesh-like expanded grids. This expanding method includes a process of continuous application of an active material to the sheet with mesh-like expanded grids and a process of cutting the sheet to specified dimensions.
Thus, the expanding method employed in the production of electrodes has shown such advantages as excellent productivity and easiness in reducing the thickness of electrodes by changing the thickness of sheet material.
Conversely, the expanding method has shown such a drawback as having a large strain created and remained at each respective connecting section of the mesh-like expanded grids due to the distinctions thereof, resulting in local corrosion caused to the positive electrodes that have used the electrodes thus prepared. However, it is also possible to produce grids that show almost the same resistance to corrosion as a grid produced by the casting method as a result of improvement in alloys.
By the use of grids produced with this expanding method it is possible to reduce the thickness of electrodes, thereby enabling an improvement in batteries' output characteristics.
Since there are no strand with this expanded grid as with a grid prepared according to the casting method, the batteries' output characteristics are extensively degraded in the domains of large depth of discharge, where conductivity of an active material itself is lost, thus resulting in a difficulty in maintaining a specified high output over a wide range of depth of discharge. This is attributed to the negative electrodes. In connection to high rate discharge cycle life characteristics, corrosion occurs in the grids of positive electrodes according to the lapse of cycles, further often causing the batteries' life to be shortened due to sulfation of the active material in negative electrodes.
The present invention provides a lead-acid storage battery, in which electrodes using grids prepared by an expanding method show excellent output characteristics even towards the end of discharge and also excellent high rate discharge cycle life characteristics.